3.1 AMMI ANOVA
The ANOVA on the yields of the 12 foxtail millet cultivars in eight environments is shown in Table 1. G, E, and G×E interaction reached significant levels based on AMMI ANOVA (p < 0.01). Specifically, G, E, and G×E interaction contributed 19.5%, 43.4%, and 28.6% of the total sum of squares, respectively. This finding suggests that GEI affects the yield performance of cultivars across environments. Therefore, performing GEI analysis on cultivars based on AMMI is necessary. The G×E interaction was further decomposed into IPCA1-IPCA4 and residual effects. The decomposition results showed that the first three principal components were highly significant. PCA1, PCA2, and PCA3 accounted for 63.0%, 14.9%, and 9.9% of the sum of squares of G×E interaction, respectively. However, the residual only accounted for 5.9%. These results show that AMMI can effectively evaluate the stability of cultivars since the first three principal components explained 87.9%.
Table 1
AMMI ANOVA for grain yield (kg ha− 1) of foxtail millet tested across eight environments
Source | df | SS | MS | %SS | %GE | F | p-value |
Total | 191 | 526844829 | 2758350 | | | | |
Treatments | 95 | 482201301 | 5075803 | 91.5 | - | 122.95 | < 0.001 |
Genotypes | 11 | 102561810 | 9323801 | 19.5 | - | 225.85 | < 0.001 |
Environments | 7 | 228767771 | 32681110 | 43.4 | - | 6.38 | < 0.001 |
Block | 8 | 41010568 | 5126321 | 7.8 | - | 124.17 | < 0.001 |
Interactions | 77 | 150871720 | 1959373 | 28.6 | - | 47.46 | < 0.001 |
IPCA1 | 17 | 95116387 | 5595082 | - | 63.0 | 135.53 | < 0.001 |
IPCA2 | 15 | 22525315 | 1501688 | - | 14.9 | 36.37 | < 0.001 |
IPCA3 | 13 | 14891571 | 1145505 | - | 9.9 | 27.75 | < 0.001 |
IPCA4 | 11 | 9493387 | 863035 | - | 6.3 | 20.90 | |
Residuals | 21 | 8845060 | 421193 | | 5.9 | 10.20 | |
Error | 88 | 3632959 | 41284 | | | | |
NB: The block source of variation refers to blocks within environments. df, degree of freedom; SS, sum of squares; MS, mean sum of squares. |
According to AMMI 1, high-yield cultivars or environments were distributed in quadrants I and IV, while low-yield ones were distributed in quadrants II and III (Fig. 1). Therefore, ZJK and HEHT are high-potential environments, while CF and LZ are low-potential environments. Similarly, cultivars ZZG21, CG26, and YG35 are specifically adapted to high-potential environments to some extent. In contrast, cultivars FGZ19 and Y14-31 are specifically adapted to low-potential environments. IPCA1 scores reflect stability in some degree, an IPCA1 score of approximate 0 indicates that the cultivar has a relatively weak interaction relationship with environments (stable). Thus, cultivars YG35, FH9, DT29, and DT48 might be the most stable genotypes among the range of environments based on IPCA1. However, IPCA2 also significantly affects G×E. Therefore, the IPCA1 scores were plotted against IPCA2 further to analyze the stability of cultivars. It was found that YG35 and DT29 were outliers, suggesting that they significantly respond to the second principal component (Fig. 2). Classifying YG35 and DT29 as stable cultivars was correct based on IPCA1 alone, but it was incorrect based on IPCA1 and IPCA2. Therefore, FH9, ZZG21, and DT48 were stable and high-yield cultivars for all environments.
In AMMI II, each environment was linked with the origin by a line. The length of the environmental vector denotes the discriminatory ability of each environment. In Fig. 2, XJ and LZ were farthest from the origin, suggesting that the two environments had the best discriminatory ability but were not representative (unstable). In contrast, YL, CF, and GY were nearest to the origin, suggesting they were the most representative (stable) but with a relatively weak discriminatory ability. Other environments had a general discriminatory ability for cultivars.
The ranking orders of the yielding stability of cultivars were evaluated according to the AMMI-based stability parameters (Table 2). In this study, six stability parameters were selected. The corresponding cultivar was more stable when any of these stability parameters had a value closer to 0. Therefore, for ASV, the cultivars were ranked in the following descending order of stability: YG35, FH9, DT29, ZZG21, DT48, 022 − 4, FYG-4, YG38, DT45, CG26, Y14-31, and FGZ19. For ASI, YG35, FH9, DT29 were relatively stable cultivars, while CG26, Y14-31, and FGZ19 had the lowest stability. YG35, with the lowest AVAMGE value of 3,357.7, was the most stable cultivar. However, FGZ19, with the highest value of 7,768.4, was the least stable. YG35, FH9, and DT29, with the lowest MASI values of 6.7, 7.5, and 10.6, respectively, were relatively stable cultivars. Similarly, for MASV/WAAS, YG35, DT29, and ZZG21/YG35, FH9, and DT29 were relatively stable. FGZ19, with the highest MASI, MASV, and WAAS values (6.7, 59.0, and 10.0, respectively), was the most unstable cultivar. However, YG35, ZZG21, and DT29, with GSI values of 4, 5, and 7, respectively, were the stable and high-yield cultivars according to yield and stability parameter GSI.
Table 2
Mean yield (kg ha− 1), ranking, and AMMI-based stability indices parameters and rankings
GEN | Yield (kg ha− 1) | Y_R | ASI | ASI_R | ASV | ASV_R | AVAMGE | AVAMGE_R | MASI | MASI_R | MASV | MASV_R | WAAS | WAAS_R | GSI |
ZZG21 | 7674.5 | 1 | 10.6 | 4 | 71.1 | 4 | 4,461.9 | 3 | 10.7 | 4 | 94.8 | 3 | 14.5 | 4 | 5 |
CG26 | 6620.5 | 2 | 17.8 | 10 | 119.3 | 10 | 6,946.3 | 10 | 18.2 | 11 | 139.9 | 10 | 23.7 | 11 | 12 |
YG35 | 6353.8 | 3 | 6.7 | 1 | 45.1 | 1 | 3,357.8 | 1 | 6.7 | 1 | 59.0 | 1 | 10.0 | 1 | 4 |
DT29 | 6197.6 | 4 | 10.6 | 3 | 70.7 | 3 | 5,458.5 | 8 | 10.6 | 3 | 80.5 | 2 | 14.5 | 3 | 7 |
022 − 4 | 5957.5 | 5 | 15.5 | 6 | 103.6 | 6 | 5,024.6 | 6 | 15.5 | 6 | 117.4 | 6 | 18.4 | 6 | 11 |
FH9 | 5867.4 | 6 | 7.4 | 2 | 49.5 | 2 | 4,347.6 | 2 | 7.5 | 2 | 101.7 | 5 | 11.4 | 2 | 8 |
FYG4 | 5529.5 | 7 | 16.2 | 7 | 108.3 | 7 | 7,157.4 | 11 | 16.4 | 7 | 124.5 | 8 | 22.9 | 10 | 14 |
DT48 | 5517.8 | 8 | 11.4 | 5 | 76.5 | 5 | 4,886.1 | 5 | 11.5 | 5 | 96.6 | 4 | 14.5 | 5 | 13 |
DT45 | 5477.4 | 9 | 17.0 | 9 | 114.1 | 9 | 6,667.6 | 9 | 17.0 | 9 | 144.3 | 11 | 20.9 | 8 | 15 |
YG38 | 5371.3 | 10 | 16.4 | 8 | 110.1 | 8 | 4,581.6 | 4 | 16.5 | 8 | 117.9 | 7 | 19.9 | 7 | 18 |
Y14-31 | 5141.7 | 11 | 18.0 | 11 | 120.8 | 11 | 5,059.9 | 7 | 18.1 | 10 | 130.2 | 9 | 22.3 | 9 | 22 |
FGZ19 | 4835.8 | 12 | 25.6 | 12 | 171.8 | 12 | 7,768.4 | 12 | 25.6 | 12 | 186.1 | 12 | 29.4 | 12 | 24 |
The correlation between stability parameters was then determined (Table 3), and significant correlations were found in pairwise comparisons, which indicating that the six stability parameters had similar stability. The first four cultivars suitable for each environment were then analyzed, and ZZG21 was the first-choice cultivar suitable for five environments and the second-choice cultivar suitable for the other three environments (Table 4), showing the superiority of hybrid foxtail millet.
Table 3
Correlation of six stability parameters based on AMMI
Stability indexes | ASI | ASV | AVAMGE | MASI | MASV | WAAS |
ASI | 1.00** | | | | | |
ASV | 1.00** | 1.00** | | | | |
AVAMGE | 0.80** | 0.80** | 1.00** | | | |
MASI | 1.00** | 1.00** | 0.80** | 1.00** | | |
MASV | 0.94** | 0.94** | 0.83** | 0.94** | 1.00** | |
WAAS | 0.98** | 0.98** | 0.86** | 0.98** | 0.93** | 1.00** |
NB: **: significant at p < 0.01. |
Table 4
The first four AMMI selections of foxtail millet per environment
Number | Environment | Mean | Score | 1 | 2 | 3 | 4 |
1 | CF | 4216 | 21.17 | CG26 | ZZG21 | YG35 | FH9 |
2 | DT | 6742 | -0.37 | YG35 | ZZG21 | 022 − 4 | DT48 |
3 | GY | 6203 | 13.04 | ZZG21 | YG35 | CG26 | DT48 |
4 | HEHT | 7381 | -17.22 | ZZG21 | YG35 | DT45 | 022 − 4 |
5 | LZ | 4710 | -66.7 | ZZG21 | DT29 | 022 − 4 | FGZ19 |
6 | XJ | 5198 | 21.96 | CG26 | ZZG21 | DT29 | FYG4 |
7 | YL | 5427 | -4.03 | ZZG21 | DT29 | FYG4 | YG35 |
8 | ZJK | 7153 | 32.15 | ZZG21 | CG26 | YG35 | FYG4 |
3.2 Adaptability of tested cultivars
The biplot (Fig. 3) shows the “which-won-where” pattern of test data in multiple environments, and the x-axis PC1 and y-axis PC2 explained 44.20% and 37.29% of the total variation (81.49%). The cultivars farthest from the origin in various directions formed a polygon, encompassing all the other cultivars. The polygon was divided into four sectors when a vertical line was drawn from the origin to each side of the polygon, dividing each test site into two sectors. The eight environments were divided into two groups (ZJK as one group and the other seven environments as the second group), and suitable cultivars were then identified for each group.
The cultivar at the vertex angle of a sector was the cultivar with the highest yield in various test sites. Conversely, for cultivars inside the polygon, the one closer to the origin had a yield closer to the mean yield and lower sensitivity to environmental changes. The tested cultivars at vertex angles included ZZG21, CG26, Y14-31, and FGZ-19. ZZG21 belonged to sectors DT, LZ, YL, HEHT, CF, GY, and XJ, suggesting that ZZG21 has the strongest yielding ability. In contrast, CG26 belonged to only one sector, suggesting that CG26 performs best only in ZJK. The performance of other cultivars was not ideal in any test site.
3.3 Representativeness and discriminatory ability of test sites
The similarity between environments in cultivar evaluation was analyzed by drawing a line segment from the center to each environment (Fig. 4), and the cosine of the angle between two line segments reflected the correlation between two environments. An angle < 90° and > 90° indicated positive and negative correlations, respectively, while 90° indicated no correlation. In this experiment, a high correlation was observed between ZJK, XJ, CF, and GY, between DT and YL, and between HEHT and LZ.
In Fig. 5, the arrow line passing through the origin represents an average environment axis (AEA). The direction of the AEA arrow is a comprehensive evaluation of the representativeness and discriminatory ability of the test site. The length of the line segment from the origin to the test site denotes the discriminatory ability of the test site for cultivars. The longer the line segment, the stronger the discriminatory ability of the test site. The angle between the environmental vector and AEA denotes the representativeness of the test site. The smaller the angle, the stronger the representativeness of the test site. LZ, XJ, and CF had a relatively strong discriminatory ability, followed by ZJK, HEHT, GY, and YL, while DT had a relatively weak discriminatory ability. DT and YL had the strongest representativeness, followed by GY, HEHT, CF, and XJ, with moderate representativeness. ZJK and LZ had the least representativeness. A test site with strong discriminatory ability and weak representativeness can eliminate unstable cultivars, but not for selecting superior cultivars. An ideal test site should have a strong discriminatory ability for cultivars and strong representativeness for ecotopes. Herein, CF and XJ were the ideal test sites.
3.4 Yielding ability and stability of tested cultivars
The top-ranking and stable cultivars were identified using GGE biplot through average environment coordinate (AEC). The yielding ability and stability of cultivars were assessed by drawing a vertical line from each tested cultivar to AEA. The arrow of AEA pointed to the trend under which a cultivar approaches the mean yield in all environments. The vertical line drawn from a cultivar to AEA denoted the stability of G×E interaction. The longer the vertical line, the more unstable the cultivar. It can be seen that ZZG21 had the highest yielding ability, followed by CG26, DT29, YG35, FH9, 022 − 4, FYG4, DT48, DT45, YG38, and Y14-31, while FGZ19 had the lowest yielding ability. Moreover, YG35, FH9, DT29, and ZZG21 had the highest yielding stability, while FGZ19 and CG26 had the lowest yielding stability, and other cultivars had general stability. Thus, ZZG21, DT29, YG35, and FH9 were the stable and high-yield cultivars (Fig. 6).